Comparing modified substrate-induced respiration with selective inhibition (SIRIN) and N 2 O isotope approaches to estimate fungal contribution to denitrification in three arable soils under anoxic conditions

[摘要] The coexistence of many N 2 O production pathways in soil hampers differentiation of microbial pathways. The question of whether fungi are significant contributors to soil emissions of the greenhouse gas nitrous oxide ( N 2 O ) from denitrification has not yet been resolved. Here, three approaches to independently investigate the fungal fraction contributing to N 2 O from denitrification were used simultaneously for, as far as we know, the first time (modified substrate-induced respiration with selective inhibition (SIRIN) approach and two isotopic approaches, i.e. end-member mixing approach (IEM) using the 15 N site preference of N 2 O produced ( SP N 2 O ) and the SP/ δ 18 O mapping approach (SP/ δ 18 O Map)). This enabled a comparison of methods and a quantification of the importance of fungal denitrification in soil. Three soils were incubated in four treatments of the SIRIN approach under anaerobic conditions to promote denitrification. While one treatment without microbial inhibition served as a control, the other three treatments were amended with inhibitors to selectively inhibit bacterial, fungal, or bacterial and fungal growth. These treatments were performed in three variants. In one variant, the 15 N tracer technique was used to estimate the effect of N 2 O reduction on the N 2 O produced, while two other variants were performed under natural isotopic conditions with and without acetylene. All three approaches revealed a small contribution of fungal denitrification to N 2 O fluxes ( f FD ) under anaerobic conditions in the soils tested. Quantifying the fungal fraction with modified SIRIN was not successful due to large amounts of uninhibited N 2 O production. In only one soil could f FD be estimated using modified SIRIN, and this resulted in 28  ±  9 %, which was possibly an overestimation, since results obtained by IEM and SP/ δ 18 O Map for this soil resulted in f FD of below 15 % and 20 %, respectively. As a consequence of the unsuccessful SIRIN approach, estimation of fungal SP N 2 O values was impossible. While all successful methods consistently suggested a small or missing fungal contribution, further studies with stimulated fungal N 2 O fluxes by adding fungal C substrates and an improved modified SIRIN approach, including alternative inhibitors, are needed to better cross-validate the methods.

[学科分类] 大气科学